Nowadays, different alloys are known for their capacity of reversible hydrogen storage which may be divided into two groups: alloys with absorption/desorption temperatures lower than 100° C. (373 K) at atmospheric pressure and hydrogen storage capacity up to 2 wt. %, such as reported by Sapru et al., in U.S. Pat. No. 6,616,891, September 2003; said alloy with chemical composition consisting of Ti(30-35 at. %)—V(25-30 at. %)—Cr(25-30 at. %)—Mn(10-15 at. %) has a reversible hydrogen desorption capacity over 2.8 wt. %, at a temperature close to 150° C.; and by Lee et al., in U.S. Pat. No. 5,888,317, March 1999, said alloy with chemical composition consisting of Ti—Zr—Mn—Cr—V—X, wherein X (Fe, Al or Ni) has a hydrogen storage capacity of 2.03 wt. % at 30° C. (303 K); and alloys with hydrogen absorption/desorption temperatures higher than 250° C. at atmospheric pressure and hydrogen storage capacity greater than 3 wt. %. The magnesium based-alloys are included within this group. Magnesium, as magnesium hydride (MgH2), has the greatest theoretical hydrogen storage capacity, 7.6% in weight basis, among all metals.
However, magnesium has some shortcomings for practical use as a hydrogen storage medium, such as: slow hydrogen absorption/desorption kinetics, poor ability for dissociating the hydrogen molecules on its surface and the high stability of the magnesium hydride, because of strong bonds between magnesium and hydrogen atoms. Additionally, the activation process in magnesium based-alloys is complex, as mentioned by Ovshinsky et al., in U.S. Pat. No. 6,746,645, June 2004, which referenced the U.S. Pat. No. 3,479,165, November 1969, this latter having disclosed that for eliminating surface barriers it is necessary to use temperatures between 400 and 425° C. (673 and 698 K) at pressures close to ˜7 MPa for several days.
Different alternatives for trying to resolve the problems associated to magnesium and its alloys have been proposed. Such alternatives include the following: 1) Sapru et al., in U.S. Pat. No. 6,103,024, August 2000, used mechanical alloying to produce the Mg(75-95 at. %)—Ni(5-15 at. %)—Mo(0.5-6 at. %) alloy, with addition of at least one element among Al, C, Ca, Ce, Co, Cr, Cu, Fe, Dy, La, Mn, Nd, Si, Ti, V and Zr, with a concentration between 1-15 at. %, said alloy has a hydrogen storage capacity between 3.2 and 5.7 wt. % at 300° C. (573 K); 2) Ovshinsky et al., in U.S. Pat. No. 4,431,561, February 1984, modified magnesium structurally and chemically through the “sputtering” technique with addition of one or more elements from the group of elements consisting of C, O, Fe, Al and Cu; for example, the Mg—C—O—Cu alloy reached a hydrogen storage capacity of 5.9 wt. % at 300° C. (573 K); 3) Schulz et al., in U.S. Pat. No. 5,964,965, October 1999, produced alloys by mechanical grinding or mechanical alloying in nanocrystalline powder form, satisfying the chemical composition M1-xAxDy, wherein, M: Mg or Be; A: preferably Zr, Ti, Ni; D: preferably Pd; x in the range between zero and 0.3, and y in the range between zero and 0.15; which reached hydrogen absorption capacity as high as 6.6 wt. % at 230° C. (503 K); 4) Ovshinsky et al., in U.S. Pat. No. 6,746,645, June 2004, produced a magnesium alloy in the form of fine particles with chemical composition Mg(90 wt. %)—Ni(0.5-2.5 wt. %)-Mm(1-4 wt. %), wherein Mm comprises predominantly Ce, La, Pr, and with addition of one or more elements from the group consisting of Al, Y and Si, presented a hydrogen storage capacity over 6 wt. % at 300° C. (573 K), and a life cycle of at least 500 cycles without loss of initial capacity; 5) Young et al., in U.S. Pat. No. 6,726,783, April 2004, fabricated alloys of chemical composition Mg(90 wt. %)—Ni(0.5-2.5 wt. %)-Mm(1-5.5 wt. %), where Mm consists predominantly of Ce, La, Pr, Nd, and further including one or more elements from the group of Al, Y, B, C, Si, preferably produced by gas atomization with particle size between 30 and 70 μm, which presented a hydrogen storage capacity well over 6 wt. %, and hydrogen absorption kinetics such that 80% of its total capacity was absorbed within five minutes at 300° C.
As mentioned above, important advances with respect to hydrogen properties in the magnesium based-alloys have been achieved, with marked attention in the adjustment of chemical composition using transition elements; which, due to its chemical nature, can modify the character and the strength of the ionic and/or covalent bonds between magnesium and hydrogen atoms, to reduce the high magnesium hydride thermodynamic stability. Improvements also have been made on fabrication techniques.
The magnesium based-alloys of this invention resulted from the combination of factors such as the addition of transition and rare earth elements to the magnesium matrix, and the use of fabrication techniques as induction melting and rapid solidification.
The alloying elements were selected in accordance to the following premises:
1) the stability of the magnesium hydride is influenced by changes in the crystallographic characteristics and in the nature of the chemical bonds. The addition of transition elements such as Ti (3d), Fe (3d), Ni (3d), Cu (3d), Nb (4d) causes changes in the unit cell volume. For example, the addition of aluminum is associated to an increase of 46.21 kJ/mol H2 in the total energy of the MgH2—Al system, which is reflected on the weakening of Mg—H bonds, improving the hydrogen storage properties, as mentioned by Song, Y. et al., in “Influence of Selected Alloying Elements on the Stability of Magnesium Dihydride for Hydrogen Storage Applications: A First-Principles Investigation,” Physical Review, B 69, pp. 1-11, 2004. On the other hand, Chen et al., in “Alloying Effects of Transition Metals on Chemical Bonding in Magnesium Hydride MgH2”, Acta Materialia, 52, pp. 521-528, 2004, evidenced the effect of 3d and 4d transition elements on the electronic structure of magnesium hydride. Thus, elements such as Zr (4d), Y (4d) and Sc (3d) reduce the MgH2 thermodynamic stability, possibly due to their effect on increasing unit cell volume.
Some catalytic transition elements, as for example, Fe (3d), Ni (3d) and Co (3d), have the ability of improving the hydrogen absorption/desorption kinetics. For instance, in the hydrogenation process and due to the good Ni mobility in Mg, it is possible that a stable second phase, Mg2NiH4, could be formed. This phase presents a heat of formation of 62.7 kJ/mol H2, which is larger than that of MgH2, so, the hydrogen atoms in the mixture of MgH2 and Mg2NiH4 compounds can be released more readily than those in solely MgH2. On the other hand, nickel has a high dissociation capacity of the hydrogen molecule.
Rare earth and some transition elements act as catalysts for the dissociation of hydrogen molecules. They assist the hydrogen absorption/desorption reactions improving the hydriding/dehydriding kinetics. Yamada et al., in “Hydrogen Storage Properties and Phase Structures of Mg Rich Mg—Pd, Mg—Nd and Mg—Pd—Nd Alloys”, Materials Transactions, 42, pp. 2415-2421, 2001, and Yin et al., in “Hydrogen Storage Properties and Structure Characterization of Melt-Spun and Annealed Mg—Ni—Nd Alloy”, Materials Transactions, 43, pp. 417-420, 2002; have shown that the addition of rare earth elements to magnesium, specifically Nd, improves the hydrogen absorption and desorption kinetics, due to the catalytic action of the neodymium hydride on the dissociation or on the recombination of the hydrogen molecule.
The following premises were adopted concerning the fabrication procedures:
1) rapid solidification is a melting process that presents advantages such as: metallic ribbons with thickness in the range from 30 to 200 μm can be produced, which will be beneficial to improve the hydrogen absorption/desorption kinetics. Problems associated to precipitation of near-surface hydride phase can be reduced and as mentioned by Pedersen et al., in “The Formation of Hydride in Pure Magnesium Foils”, Journal of the Less Common Metals, 131, pp. 31-40, 1987, magnesium foils thicker than 60 μm may remain with some non-reacted metallic magnesium after hydrogenation.
2) also, depending on the alloy's chemical composition and the operation parameters it is possible to produce amorphous and/or nanocrystalline microstructures by rapid solidification. Amorphous and/or nanocrystalline microstructures present a large number of interfaces and grain boundaries that promote the absorption of hydrogen, providing easy pathways for hydrogen diffusion. Thus, in a microstructure formed by amorphous and nanocrystalline phases, the amorphous phase leads to an easier access of hydrogen to the nanograins, avoiding the diffusion of hydrogen through the already formed hydride, as mentioned by Spassov, T. et al., in “Hydrogenation of Amorphous and Nanocrystalline Mg-Based Alloys” Journal of Alloys and Compounds, 287, pp. 243-250, 1999. Thin ribbons of magnesium-based alloys are less prone to undergo decrepitation upon hydriding.